The growing popularity of applications such as digital television, DVD-video and DVD-audio is causing multi-channel audio sources to become more common in consumer audio playback systems. This growth in popularity is reflected in the growing number of consumer playback systems that are capable of reproducing three or more channels of audio information. Consumer systems with five full-bandwidth channels and a low-frequency effects (LFE) channel such as those used in home-theatre applications are becoming increasingly common. This particular arrangement is sometimes referred to as 5.1 channels. In spite of this trend, one- and two-channel audio sources such as compact disc (CD) players, MP3 players, conventional analog and digital radio receivers and conventional television receivers are still commonly used and are likely to be widely used for many more years.
As consumers become familiar with the aural realism and impact that is possible from systems with three or more channels, they begin to expect and demand similar performance from system components that provide only one and two channels of audio information. The reproduction of a two-channel audio program through only two channels of a system with more than two channels is becoming unacceptable to a broader range of consumers.
Techniques are known that can expand one- and two-channel signals into a larger number of channels. Products that incorporate Dolby Pro Logic ® II and Dolby Pro Logic IIx technologies of Dolby Laboratories, Inc., San Francisco, Calif., use “upmixing” to expand a two-channel signal into three or more channels of audio information. These products allow a consumer to play back two-channel audio material through a system having three or more channels with an aural experience that is similar to that provided by the playback of audio material that originated from a source having three or more channels. The proper operations of these known techniques relies on two conditions.
The first condition for proper operations is that the number of channels for the audio source must be known. Devices that incorporate Dolby Pro Logic II technology, for example, are designed to operate properly only with two-channel input. Devices that incorporate Dolby Pro Logic IIx technology can operate properly with two and 5.1 input channels but the number of input channels must be known because their operation varies according to the number of input channels. For many applications, this condition can be met easily either because the number of channels is known implicitly or because it is conveyed explicitly with the audio information. For example, it is known implicitly that two channels of audio information are provided by audio sources such as cassette tape decks, CD players and FM-stereo broadcast receivers. Other sources provide signals, such as television signals with encoded audio information conforming with the Advanced Television Systems Committee (ATSC) A/52 specification, that convey “metadata” explicitly specifying the number of channels.
The second condition for proper operation is that all channels in the source must be active; i.e., no channel of the audio source can be silent at all times. For example, if an audio source delivers 5.1 channels of audio information to a receiver with a Dolby Pro Logic IIx decoder and all of the channels except for the left and right channels are muted, the receiver will incorrectly configure the decoder processor and fail to deliver active signals to all of its output channels. Although this situation may not arise often in broadcast situations, it is typical of conditions that exist in computer systems with audio and multi-media capabilities.
The use of computers as sources of audio information in consumer entertainment systems is becoming more common. Special purpose hardware and software allow an otherwise conventional personal computer to operate as a CD player or DVD player for audio and video, a video-game console, a digital television receiver and a music synthesizer to name only a few examples. Many of these sources provide five or more channels of audio information while others provide only two channels.
Within the computer itself, software applications typically deliver their audio output to a common mixer that is capable of combining audio information from several sources and presenting the combined result to an output device such as a so called “sound card” or other output device. Output signals from this device can be provided to an acoustic output transducer such as headphones or to an amplifier that drives one or more loudspeakers, or they can be provided to other hardware or software devices for subsequent processing.
In environments such as that found in computers running one of the Windows operating systems available from Microsoft Corporation, Redmond, Wash., the mixing function is provided by a component of the operating system or by a special-purpose driver that is installed to support a particular sound card or other output device. The number of output channels supported by this mixing function typically depends on the number of channels that are supported by the output device. If the output device is limited to two channels, the mixing function provides two output channels. If the output device supports 5.1 channels, the mixing function provides 5.1 output channels. In typical installations, the number of output channels and the mixing process of the mixing function cannot be adjusted. This situation presents limitations that cannot be overcome by known techniques.
For example, suppose a computer system has a sound card that supports 5.1 output channels and two audio sources. In principle, either one or both of these sources may be implemented by hardware and software within the computer system or by devices that are external to the computer. In this example, the first source is a CD-player that provides two channels of audio information that are configured as left and right channels and the second source is a video game that provides 5.1 channels of audio information configured as left, right, center, left-surround, right-surround and low-frequency effects (L, R, C, LS, RS, LFE) channels. A typical mixing function in the computer mixes the respective input channels together. The left-channel signals from all sources are mixed and provided at a left-channel output of the mixer. The right-channel signals from all sources are mixed and provided at a right-channel output of the mixer. Similarly the C, LS, RS and LFE channel signals from all sources are mixed and provided at respective outputs. In this example, however, only left channel and right channel signals from both sources are mixed because only one of the sources provides C, LS, RS and LFE channel signals. A consumer would hear audio from the CD player through only two channels of the system but would hear audio from the video game through all channels. As mentioned above, consumers are coming to expect and demand that the audio from all sources be presented through all channels.
What is needed is a facility that overcomes this limitation of the prior art.